A number of prior patents disclose antennas capable of operation to provide varying electromagnetic wave propagation.
U.S. Pat. No. 3,560,978 discloses an electronically controlled antenna system comprising a monopole radiator surrounded by two or more concentric circular arrays of parasitic elements which are selectively operated by digitally controlled switching diodes. In the antenna system of U.S. Pat. No. 3,560,978, recirculating shift registers are used to inhibit the parasitic elements in the circular arrays to produce the desired rotating wave pattern.
U.S. Pat. No. 3,877,014 relates to an electronically scanned, multiple element antenna array in combination with means for changing its operation between a multiple element array and an end-fire mode of operation. In the antenna of U.S. Pat. No. 3,877,014, a transmitter is switched to feed either a column array of antenna elements or the end-fire feed element. During end-fire operation, the column array of antenna elements are short circuited.
U.S. Pat. No. 3,883,875 discloses a linear array antenna adopted for commutation in a simulated Doppler ground beacon guidance system. In the end-fire commutated antenna array of U.S. Pat. No. 3,883,875, the linear array of n radiator elements is combined with a transmitting means for exciting each of the n-1 of said elements in turn, and an electronic or mechanical commutator providing for successive excitation in accordance with the predetermined program. Means are provided for short circuiting and open circuiting each of the n-1 elements, and the short circuiting and open circuiting means is operated in such a manner that during excitation of any one of said elements, the element adjacent to the rear of the excited elements operates as a reflector and the remaining n-2 elements remain open circuited and therefore electrically transparent. A permanently non-excited element is located at one end of the array.
In "Reactively Controlled Directive Arrays", IEEE Transactions on Antennas and Propagation, Vol. A-26, No. 3, May, 1978, Roger F. Harrington discloses that the radiation characteristics of an n-port antenna system can be controlled by impedance loading the ports and feeding only one or several of the ports. In Harrington's disclosed system, reactive loads can be used to resonate a real port current to give a radiation pattern of high directivity. As examples of the system, Harrington discloses a circular array antenna with six reactively loaded dipoles equally spaced on a circle about a central dipole which is fed, and a linear array of dipoles with all dipoles reactively loaded and one or more dipoles excited by a source. In operating the circular array antenna, Harrington discloses that by varying the reactive loads of the dipoles in the circular array, it is possible to change the direction of maximum gain of the antenna array about the central fed element and indicates that such reactively controlled antenna arrays should prove useful for directive arrays of restricted spatial extent.
U.S Pat. No. 4,631,546 discloses an antenna which has a transmission and reception pattern that can be electrically altered to provide directional signal patterns that can be electronically rotated. The antenna of U.S. Pat. No. 4,631,546 is disclosed as having a central driven antenna element and a plurality of surrounding parasitic elements combined with circuitry for modifying the basic omni-directional pattern of such an antenna arrangement to a directional pattern by normally capacitively coupling the parasitic elements to ground, but on a selective basis, changing some of the parasitic elements to be inductively coupled to ground so they act as reflectors and provide an eccentric signal radiation pattern. By cyclically altering the connection of various parasitic elements in their coupling to ground, a rotating directional signal is produced.
U.S. Pat. No. 4,700,197 discloses a small linearly polarized adaptive array antenna for communication systems. The antenna of U.S. Pat. No. 4,700,197 consists of a ground plane formed by an electrically conductive plate and a driven quarter wave monopole positioned centrally within and substantially perpendicular to the ground plane. The antenna further includes a plurality of coaxial parasitic elements, each of which is positioned substantially perpendicular to but electrically isolated from the ground plane and arranged in a plurality of concentric circles surrounding the central driven monopole. The surrounding coaxial parasitic elements are connected to the ground plane by pin diodes or other switching means and are selectively connectable to the ground plane to alter the directivity of the antenna beam, both in the azimuth and elevation planes.
U.S. Pat. No. 3,109,175 discloses an antenna system to provide a rotating unidirectional electromagnetic wave. In the antenna system of U.S. Pat. No. 3,109,175, an active antenna element is mounted on a stationary ground plane and a plurality of parasitic antenna elements are spaced along a plurality of radii extending outwardly from the central active antenna element to provide a plurality of radially extending directive arrays. A pair of parasitic elements are mounted on a rotating ring, which is located between the central active antenna element and the radially extending active arrays of parasitic elements and rotated to provide an antenna system with a plurality of high gain radially extending lobes.
In addition, U.S. Pat. Nos. 3,096,520, 3,218,645, and 3,508,278 disclose antenna systems comprising end-fire arrays.
Antenna systems including multiple active antenna elements with phasing electronics and/or phased transmitters are disclosed, for example, in U.S. Pat. Nos. 3,255,450, 3,307,188, 3,495,263, 3,611,401, 4,090,203, 4,360,813 and 4,849,763.
Antennas comprising a plurality of antenna elements in a planar array are also known. For example, U.S. Pat. No. 4,797,682 discloses a phased array antenna structure including a plurality of radiating elements arranged in concentric rings. In the antenna of U.S. Pat. No. 4,797,682, the radiating elements of each concentric ring are of the same size, but the radiating elements of different rings are different sizes. By varying the size of the radiating elements, the position of the elements will not be periodic and the spacing between adjacent rings will not be equal. Thus, grating lobes are minimized so they cannot accumulate in a periodic manner.
Notwithstanding this extensive developmental effort, problems still exist with multiple element antenna arrays, particularly with the performance of large apertures steered to end fire.
For a beam to be formed across the upper surface of an antenna array such as that shown in U.S. Pat. No. 4,797,682, each radiating element must be capable of delivering power across the face of the array, ultimately radiating along the ground plane and into free space at the horizon. In large antenna arrays consisting of a plurality of antenna elements and having diameters in excess of 10 wavelengths, the elements will receive much of this power, and act like a very glossy surface. In short, such large arrays tend to re-absorb a large portion of the power that is intended to be radiated. This effect is well known, and is often described in terms of mutual coupling effects, or active array reflection coefficient.
The plot in FIG. 1 describes one of the results of a 1983 Lincoln Labs study of phased arrays with wire monopole radiating elements. Gain-referenced patterns are plotted for a single central element embedded in many sizes of square arrays on an infinite ground plane. FIG. 1 indicates that the horizon gain of a single element falls drastically as the size of the array increases. For a 15-wavelength antenna, an element gain degradation of some 15.0 dB would be expected.
Similar results are obtained when comparing an isolated low-profile monopole, and the same element embedded in a 15 wavelength 1306-element circular array of identical low-profile monopoles. In this case, such antennas were mounted on a ground plane approximately 40 wavelengths in diameter. The maximum measured gain of the isolated element was approximately 5.15 dBil at 10.degree. above the horizon. When embedded in the center of the 1306-element array, the element had measured gain of -11.1 dBil at 10.degree. above the horizon, corresponding to 16.25 dB degradation.
Because not all elements are effected as severely as the ones measured in the center of such an array, it is difficult to make an array gain estimate. Furthermore, some degree of active matching is possible, which should marginally improve the gain. Even so, the end-fire gain of this large circular array will almost certainly not exceed 16.0 dBil, and may be as low as 13.0 dBil. Such gain is too low for the investment in apertures, and an intolerable thermal problem will result from more than 12.0 dB of RF power dissipation in the transmit mode.